Protostome: Definition, Types, and body plans

Protostome Definition

Protostomes are a clade of animals that undergo protostomy during their embryonic development.

The protostomes, together with the Deuterostomes and the Xenacoelomorpha, make up a major group of animals called the Bilateria. These are triploblastic animals that display bilateral symmetry.


In embryo development, two gametes—a sperm and an egg—fuse to form a zygote. The zygote is divided on an axis into two “poles”: the top “animal pole” and the bottom “vegetal pole”.

The mass of the zygote then divides up in a process called cleavage, resulting in a dense ball of blastomere cells, called the morula. The pattern of cleavage is one of the fundamental developmental distinctions between protostomes and deuterostomes; protostomes divide with spiral cleavage. In this process, rather than dividing on a plane parallel to the poles (as occurs in radial cleavage), the cleavage takes place on an angle, so that the blastomeres are arranged in a spiral shape.

As more cells are produced, a layer of cells called the blastoderm, surrounds a fluid- or yolk-filled cavity called the blastocoel to form the blastula. At this point, gastrulation begins, leading to formation of the germ layers—the embryonic cells which give rise to an organism’s tissues and organs. In triploblastic organisms there are three layers.

Gastrulation begins when an indentation develops in the blastula called the blastopore. Cells of the blastopore migrate inward until they reach the opposite side of the blastula, creating an inner tube called the endoderm, which gives rise to the digestive system. On the outside is the ectoderm, which gives rise to the epidermis (skin and hair) and the nervous system. The mesoderm, which ends up as connective tissue and muscles, lies in-between.

The important distinction between protostomes and deuterostomes is at the point of the blastopore. In protostomes, the blastopore develops into the mouth, and the opposite cavity develops into the anus. In deuterostomes (the group that includes vertebrates and echinoderms), the blastopore develops into the anus.

It is useful to remember that the word comes from the Greek proto- meaning “first” and stoma meaning “mouth”. Protostomes develop a “first mouth”.

Types of Protostomes

The protostomes are split into two taxonomic groups.


This group includes the annelid worms, brachiopods, bryozoa and mollusks, as well as sometimes the Platyhelminthes and rotifers.

The Lophotrocozoa grow their bodies incrementally, by extending the size of their skeletons. For example, mollusks grow larger by adding calcium carbonate to the edges of their shells.

Some have a ‘lophophore’: a specialized ring-like structure around their mouths. This allows suspension (filter) feeding by pulling in water and food particles into the mouth and to the gut.

Some commons examples of lophotrocozoa:

  • Bivalve mollusks (clams, oysters, mussels, scallops)
  • Leeches
  • Earthworms
  • Squid
  • Octopus
  • Snails and slugs


This group includes the arthropods, nematodes and tardigrades.

The ecdysozoa have a three-layered cuticle, with a soft interior and a hard exterior called an exoskeleton.

They grow periodically by shedding or ‘molting’, and then re-growing their exoskeleton through a process called ecdysis.

During embryonic development, the ecdysozoa do not undergo spiral cleavage, as in other protostomes.

Some common examples of ecdysozoa:

  • Insects (beetles, ants, flies, crickets, butterflies, fleas, cicadas, bees)
  • Crustaceans (crabs, lobsters, crayfish, woodlice, barnacles)
  • Roundworms
  • Velvet worms
  • Spiders
  • Centipedes and Millipedes

Protostome Body Plans

The protostomes can generally be classified into three different body plans all of which are bilaterally symmetrical and triploblastic.


The majority of worm-like protostomes have a well-developed coelom, a cavity inside the body which provides space for fluids to circulate. The coelom acts as a hydrostatic skeleton, which uses fluid pressure to allow movement.

Although they have similar body plans, they are differentiated according to their specialized feeding systems.

Echiura (spoon worms) are a small group of marine, segmented worms. They acquire food (and move) using a proboscis—a structure that extends in front of the mouth—which traps food particles in mucus and passes them through to the mouth with cilia.

Pripulida (penis worms) are marine worms that have a toothed throat that they turn inside out to extend, grab prey and then retract back into the body for consumption.

Nemertea (ribbon worms) have a proboscis with a barbed tip that lies inside the body just above the gut. To feed, Nemertea extend the proboscis outside the mouth and capture prey using venom and entangling.

Arthropod Body Plans

The main distinguishing feature of the arthropods is their segmented bodies, which are split into three distinct sections: the head, the thorax and the abdomen. This group includes all insects, crustaceans, arachnids and myriapods. They are the most numerous of all living creatures, making up over 80% of all described species.

They have jointed limbs (hence the name from the Greek arthro- “joint”, and pod “foot”), and a tough exoskeleton made from chitin, often hardened with sclerotin proteins. Rather than moving using a hydrostatic skeleton, arthropod muscles are directly attached to the exoskeleton. Without the need for hydrostatic movement, the coelom is reduced to a cavity that transfers fluids and provides space for the internal organs.

Mollusk Body Plan

The mollusks are mainly marine animals and include the Bivalves (clams, muscles, scallops), Cephalopods (squid, octopus, cuttlefish, nautilus), the Chitons, and the Gastropods, of which some are terrestrial snails and slugs.

The mollusk body plan is based on three main components:

  • The foot: a large muscle at the base of the animal, usually used for movement.
  • The visceral mass: the region that contains the internal organs and the external gill. Together with the foot, the visceral mass fulfills the function that is performed by the coelom in other protostomes.
  • The mantle: the layer of tissue that covers the visceral mass. In many mollusk species the mantle secretes the calcium carbonate used to form the shell or exoskeleton.

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